Led and power supply question

Matty1973 · Post by **Matty1973** » Thu Mar 24, 2011 5:31 am

Help required.

i purchased some damaged led torches from my work, the lights work cases were broken

so there are 24 5mm leds in the unit powered by 3 x AA 1.5v batteries, with 1 x 1.2Ohm resistor Brown/Red/Gold/Gold

i want to remove the leds in it and put in coloured ones i secured a batch of 100 coloured led's that the info says they run at 2-2.2v can i use these ones in place of the originals, ie just cut the old ones out and resolder in the new ones with leads?

USS Atlantis · Post by **USS Atlantis** » Thu Mar 24, 2011 5:37 am

You'll need to replace the resistor as well - up that to a 120ohm

Matty1973 · Post by **Matty1973** » Thu Mar 24, 2011 5:51 am

USS Atlantis wrote:You'll need to replace the resistor as well - up that to a 120ohm

Can you tell me why I would need to change the resistor as well? Just like to know these things

Matty1973 · Post by **Matty1973** » Thu Mar 24, 2011 5:58 am

How would I go about converting it into a 9v system? Just change the battery pack and the resistor?

USS Atlantis · Post by **USS Atlantis** » Thu Mar 24, 2011 5:01 pm

Matty1973 wrote:
USS Atlantis wrote:You'll need to replace the resistor as well - up that to a 120ohm
Can you tell me why I would need to change the resistor as well? Just like to know these things

Based on the voltage supplied and the resistor in there, it's running 3.5v LED's - I'm thinking they're White

Lower voltage LEDs need a higher value resistor to prevent overloading the LEDs - overloading can result in brighter LEDs; for as long as they last - which could be anywhere from normal lifespan (100,0000 hours) to an immediate blowout (no light at all.... ever)

USS Atlantis · Post by **USS Atlantis** » Thu Mar 24, 2011 5:03 pm

Matty1973 wrote:How would I go about converting it into a 9v system? Just change the battery pack and the resistor?

Yep - and that resistor needs to be even higher in value than just changing the LEDs to lower voltage ones

Calculations say that 2.2v LED's running off a 9v power source needs a 330 ohm resistor

Matty1973 · Post by **Matty1973** » Thu Mar 24, 2011 8:23 pm

Ok so lets clarify this

this is the current chip that i want to use, yes it had 24 5mm white bulbs on it
http://i1103.photobucket.com/albums/g48 ... /photo.jpg

i removed the bulbs and want to replace with 3mm various colour bulbs (Predominatley white though) with leads attached, i am also going to boost it up to a 9v system.

These are the voltages of the 3mm bulbs
red bulbs: 1.9 - 2.1V, 600 - 800MCD
blue bulbs: 3.2 - 3.4V, 1500 - 2000MCD
green bulbs: 3.2 - 3.4V, 6000 - 8000MCD
yellow bulbs: 1.9 - 2.1V, 800 - 1000MCD
white bulbs: 3.2 - 3.4V, 3000 - 4000MCD

What else will i need to do, what size resistor do you think, i do have some 470ohm ones already

USS Atlantis · Post by **USS Atlantis** » Fri Mar 25, 2011 4:16 am

When mixing LEDs like that, the norm is to separate then into different runs

With that board, you can't do that, so the next best thing is to use a resistor based on the LOWEST value LED - which is your Reds and Yellows

Now one problem is that to accurately calculate the needed resistor value, you need two ratings - the voltage (v) and the current draw (ma)

The math for the resistor value is

R = (Vs - Vl) / I
Vs is your source voltage - 9 in this case
Vl is the load voltage - 1.9 (reds & yellows) for this circuit
I is the current draw - I'm going to use 25ma as that's a fairly typical value for LEDs - 25ma is .025A

so -> (9-1.9)/.025 = 284

So your lowest value LED needs a 284 Ohm resistor - since that's not a standard value in the E12 or E24 groupings (classes of resistors) we pop to the next HIGHER standard value

So you'll need a 300 or 330 Ohm resistor to protect the lowest value LEDs - and it should be rated at 1/2 watt one as well

What effect does this have on the others?

They'll be dimmer - reduce the power flow to a LED and the LED dims down; you have to decide if having all the LEDs on the same board is worth losing some output from all the Blues, Greens and Whites

Matty1973 · Post by **Matty1973** » Fri Mar 25, 2011 4:20 am

Thanks Ken that's great info, I don't mind the power drop as there will be plenty in the mix to still light from the inside